Our source for the globular cluster photometry is the compilation of HST photometry published by [1]} based on observations made with the Advanced Camera for Surveys (ACS) onboard HST.
Their survey contains deep photometry in the ACS/WFC F606W and F814W bands for 67 globular clusters. In addition to the photometry for 67 globular clusters from [1]}, we also use the ACS/WFC
photometry published by [3]} for 6
globular clusters as well as the photometry for NGC 6528 from [4]} that was kindly provided to us by the authors. We finally use the photometry from [5]} for an additional 6 globular clusters
(NGC 6325, NGC 6342, NGC 6355, NGC 6380, NGC 6401 and NGC 6558). We skipped Pal 2 due to significant differential reddening and E 3 since the cluster has such a low mass that the location of its main sequence cannot be accurately determined.
This leaves us with a sample of 79 globular clusters with accurate, deep ACS/WFC photometry in the F606W and F814W bands.
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